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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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HIDDEN, FOURTH AND SMALLER GLACIERS                  163
and turns westward, finally entering the head of Russell Fiord. A similar series of stream channels, now abandoned, show where an outlet formerly went down the west side of the valley, and between that course and Beasley Creek is a high wooded hill of dissected outwash gravels, rising to considerable height on the end nearest the glacier front (PI. LXXXVHI).
The clear ice of the glacier projects farthest in the center of the valley, at the point where the stream passes down the depression between the bordering alluvial fans. On the very outermost portion of this point is some debris, staining the surface and forming debris cones and ridges. Beyond this, in front of the apparent end of the glacier, is a border of hummocky, debris-covered ice (PL LXXXVTQ with many hollows in which water stands. Kettles, recent cracks, and pools of cold water show that the buried ice extends between seven and eight hundred feet in front of the centre of the apparent end of the glacier. The d6bris-covering of this outlying portion of the glacier is in large part gravel, laid down by deposit from the glacial streams. D6bris-covered ice also projects beyond the front on both the margins of the glacier, where the lateral moraines give a covering which has checked recession. It is possible that ice also exists beneath the alluvial fans on the two margins of the valley, but no evidence of this was discovered.
The debris-covered marginal and frontal portions just described form a part of the evidence of recent notable recession. Absence of vegetation both on the mountain side above the glacier and in the flat in front of it, give further evidence of prolonged recession. The barren zone above the ice extends to a height of from 50 to 100 feet. Above this are found alder thickets, and on the west side at a still greater elevation, above 300 feet, there is spruce growth. This spruce is mature and it therefore appears that, although the glacier is now receding and has been doing so for some time, it has not expanded more than 300 feet above its present level for many years. At a distance of something more than a quarter of a mile from the glacier front, alder five or ten years old is growing, and cotton-wood and spruce trees have advanced to within half a mile of the ice front.
These facts prove that the Fourth Glacier has been slowly receding for a considerable time, and that it has not been so greatly expanded as to reach beyond its mountain valley for many years, probably for more than a half-century. That it was, at some earlier stage, far more extensive than now, is made evident by the steepness of its valley walls and the presence of eroded spurs and numerous hanging valleys with lips from 500 to 1500 feet above the valley bottoms. The steepened slope and hanging valleys extend down beyond the end of the glacier «ven to the valley mouth. We may, therefore, safely infer that during an earlier stage, probably when Russell Fiord Glacier extended out on the foreland, and Yakutat Bay Glacier to the sea, the Fourth Glacier reached well out beyond its mountain valley and expanded in a piedmont ice bulb which perhaps coalesced with the Russell Fiord Glacier. One piece of evidence bearing upon this question of former extension of Fourth Glacier, is the fact that while the glacier is not now bringing crystalline rocks, but only sedimentaries from the Yakutat group, the gravels and bowlders in . the stream bed beyond the mountain front include many crystalline rocks. The inference from this fact is that during the earlier expanded stage of the Fourth Glacier it received ice supply from a more distant source than now, in a region where crystalline rocks form the mountain walls. This source may have been at least as far back as the upper reaches of the Hidden Glacier. In all probability during this stage, the direction of ice flow in